CHAPTER 5. MODEL DEVELOPMENT (8)

Compressive Strength Model 4: All Ages Core
Strength Model

The compressive strength model 4 was
developed to provide a strength estimate for in situ strength at
all ages of a project and covers both short-term and long-term
estimates. This model provides the ability to assess the strength
development over the entire lifetime of a project and make more
realistic estimates of distress development. This necessitates the
use of both SPS and GPS section data. The short-term strength data
for this model were primarily from SPS sections, and the long-term
strength data were controlled by GPS sections. Data from SPS and
GPS sections primarily consisted of strength tests for pavement
ages less than 3 years and greater than 5 years, respectively.
However, this resulted in the use of a limited set of independent
parameters for the model. Only information available for GPS
sections could be included. For example, gradation parameters of
coarse and fine aggregates were not considered when developing
this model.This model can be expressed as follows:

Figure 154. Equation. Prediction model 4 for
fc,t.

Where:

fc,t =
Compressive strength at age t years, psi.

w/c = Water to
cement ratio.

CMC = Cementitious
materials content, lb/yd3.

uw = Unit weight,
lb/ft3.

t = Short-term age, years.

The regression statistics for this model
are presented in table 28. The model was developed using 580 data
points, and the prediction has an R2 value of
55.4 percent and an RMSE value of
992 psi. Table 29 provides details of the range of data used to
develop the model. Figure 155
and figure 156 show the predicted versus measured plot and the
residual plot, respectively. Figure 157 through figure 160 show the
sensitivity of this model to w/c ratio, CMC, unit weight, and age,
respectively. Again, the sensitivity plots showing the variation in
core compressive strength with changes in w/c ratio, CMC, and unit
weight are presented for 28 days, 1 year, and 20 years. The rate of
strength gain is much higher in the short term (28 days to 1 year)
than during the next 19 years in the long term. The use of the
logarithmic function for the age parameter is justified, as these
trends mimic actual strength gain in the field or in laboratory
cast specimens. Figure 160 can be treated as the strength gain
relationship representative of a typical mix (w/c of 0.4, cement
content of 600 lb/yd3, and unit weight of 145
lb/ft3).